1. Field of the Invention
The present invention relates to a catalyst for polymerizing an aromatic vinyl compound, and a process for producing a polymer of the aromatic vinyl compound. More particularly, the present invention relates to a catalyst and a process for producing a polymer of an aromatic vinyl compound mainly in a syndiotactic configuration with high catalytic activity.
2. Description of the Related Art
Aromatic vinyl polymers are classified into three types according to the steric configuration: namely, syndiotactic polymers, isotactic polymers, and atactic polymers. Of these three types of polymers, the syndiotactic polymers are useful as heat-resistant polymers because of their higher melting points and higher crystallization rates in comparison with other types of polymers. The syndiotactic aromatic vinyl polymers are produced, for example, by employing a catalyst composed of a titanium compound such as a titanium halide and alkoxytitanium and a reaction product of an organoaluminum compound with a condensing agent such as a aluminoxane as disclosed in Japanese Patent Application Laid-Open No. Sho 62-104818.
In polymerization of styrene, however, the catalysts prepared by combination of a titanium compound such as titanium tetrahalide and tetraethoxytitanium with methylalumoxane have low activity, and consequently the catalyst remains in a large amount in the produced polymer, which causes presumably discoloration by heat of the polymer remarkably in heat molding, rendering the polymer unsuitable for practical uses.
On the other hand, in the polymerization of styrene with a catalysts, which is prepared by combining methylalumoxane with a complex prepared by reacting a transition metal compound such as titanium tetrachloride with a diphenyl sulfide type of organic compound, although the catalyst has high polymerization activity, the ratio of stereoregular polymer is rather low in the resulting whole polymer mainly because of formation of an atactic polymer as a by-product. If the content of the atactic polymer is high, post-treatment such as solvent extraction is disadvantageoulsy required in order to avoid the adverse effects of the atactic polymer on melting point and crystallization velocity.
After comprehensive studies by the present inventors to offset the above disadvantage, it has been found that a syndiotactic aromatic vinyl polymer having a high degree of syndiotacticity can be produced with high catalyst activity by use of a specific catalyst system composed of an organic transition metal compound derived by reacting a transition metal compound with an alkylaluminum, and an organic aluminum compound. Consequently the present invention has been completed on the basis of this discovery.